1. Technical Field
The present invention relates generally to electronic circuits, and more specifically, the invention relates to switched mode power supplies.
2. Background Information
A common application of switching power supplies is to charge batteries. The output power of a battery charger is usually controlled to provide a regulated voltage and a regulated current. The voltage is regulated between a maximum and a minimum voltage over a range of output current. The current is regulated between a maximum and a minimum current over a range of output voltage. Regulation is achieved by means of a feedback signal that reduces the output of the power supply whenever the output voltage or the output current exceeds a regulated value. The power supply typically has a fault protection feature that prevents excessive output voltage or excessive output current when there is no feedback signal. Without the fault protection feature, a failure that results in the loss of the feedback signal would cause the output voltage or output current to go high enough to damage the battery or the power supply. Therefore, the absence of a feedback signal typically causes the power supply to operate in an auto-restart cycle that substantially reduces the average output voltage and average output current until the feedback signal is restored.
Typical battery chargers usually exhibit an abrupt transition from regulated output current to regulated output voltage. That is, the locus of output voltage and output current plotted in Cartesian coordinates usually has a sharp corner at the point of transition that corresponds to the point of maximum output power.
The practice of designing a battery charger to have a sharp transition between regulated voltage and regulated current can result in a product that costs more than necessary to provide the desired function. It is often possible to reduce the cost of the battery charger and to meet all requirements by designing an unregulated transition between the regulated voltage and the regulated current. The output voltage and output current in the region of unregulated transition is bounded by the natural output characteristics of the switching regulator, and typically follows the curve of maximum output power for a given output voltage and output current.
To achieve lower cost, the switching regulator is designed to operate with a control circuit that permits the regulator to make an unregulated transition between regulated output voltage and regulated output current such that the voltage and current are maintained within their specified boundaries. Proper design of the unregulated transition within the specified boundaries of output voltage and output current reduces the maximum power output, allowing the use of components that are less costly than the components to guarantee higher output power. The control circuit operates the switching regulator for regulated voltage, regulated current, or unregulated transition, depending on the current demanded by the load.
The fault protection feature that responds to the absence of the feedback signal presents an obstacle to sustained operation in the unregulated transition region. The feedback signal is substantially zero when the power supply operates in the unregulated transition region because the output voltage and output current are substantially below their regulated values. By design, the power supply operates to produce maximum output power in the unregulated transition region. The fault protection feature typically permits the power supply to produce maximum output power without a feedback signal for only the short time required to bring the output voltage or output current from zero to the regulated value under normal load conditions after start-up. If a feedback signal is not present after the normal start-up time, the power supply enters the auto-restart cycle.